Answers

Weapon power - if you look at any door or object, it will give a strength. A door might be listed as "Weak" or "Hard". A "Hard" object will take way more damage or bullets before it shatters.

Basically, never try to shoot down doors. It's a waste of ammo, and there's guaranteed to be a better solution or a better way. Either hack, use a vent to bypass the door, or use explosives if all else fails and you HAVE to get in.